Martensitic steel



United States Patent 2,999,039 MARTENSITIC STEEL Remus A. Lula, Natrona Heights, and Thomas H. Mc-

Cunn, New Kensington, Pa., assignors to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., a corp'oration of Pennsylvania No Drawing. Filed Sept. 14, 1959, Ser.N0. 839,558

5 Claims. (Cl. 148-237) This invention relates to stainless steels and in particular to martensitic type stainless steels which are age 'ha'rdenable.

Considerable research has been done in order to develop agehardenable stainless steels which are not plagued by a hypersensitive austenitic microstructure. Fabrica- .tors have expressed the desire to have a stainless steel which may be subject to severe fabrication yet be hardenable ;from relatively low heat treatment temperatures. To this end, the present invention provides for a martensitic type stainless steel which may be formed in the martensitic condition and which may be thereafter hardened at a temperature of less than 1100 -F.

An object of this invention is to provide a martensitic type stainless steel which is subject to age hardening from low heat treatment temperatures.

Another object of this invention is to provide a martensitic type stainless steel which is suitable for use in mildly corrosive atmospheres.

A more specific object of this invention is to provide a martensitic stainless steel containing carbon, manganese, silicon, chromium, nickel, molybdenum, aluminum and nitrogen as essential alloying elements, which steel ,may be hardened through an age hardening heat treat- ;ment at relativzely low temperatures.

: In its broader aspects, this :invention relates to marrtensitic type stainless steels having a composition of up to 0.04% carbon, up to 2.0% silicon, from 0.25% to "3.0% manganese, from about 7;.0% to 12.0% chromium, from about 1.0% to 6.0% nickel, from about 05% to 1.75% aluminum, up to 0.05% nitrogen, up to 5% of metal selected from the group consisting of molybdenum, 'columbium, tungsten, vanadium and titanium and the balance iron with incidental impurities. In particular, it must be noted that the steel of this invention is characterized by having a relatively low carbon and nitrogen content. As a result, the steel in the 'as-quenched condition will have amartensitic type structure and since the carbon content .is'low, the .martensitic constituent will have sufficient ductility :so that the steel may be formed in the as-quenched condition. The chromium content is -maintained within given limits for the two-fold purpose of obtaining a martensitic structure at room temperature upon quenching from the solution heat treatment temperature, yet. retain some degree of corrosion resistance comparable to the known m'artensitic type stainless steels. Aluminum performs a very vital function in that it forms a hardening precipitation which is especially useful for providing the steel with good mechanical properties. Molybdenum is effective within the steel of this invention for high temperature strength. It will be appreciated, however, "other elements, motably 'colurnbium, tungsten, vanadium "and titanium, will also perform this same ,functionand as a result, any may be substituted for ahcilybrlerrum or they may be used in combination with molybdenum. 'Nickel within the range stated is effective for providing the steel with a marte-nsitic structure upon "linenclfingxfrom .the solutionfheat treatment temperature. The balance of the steel is iron which, of course, will include the incidental impurities normally found in the furnace and had the following composition:

ICE

- .Ifreferred Range Element 1 May also include metalselected. from the, group consisting .of col-umbium, tungsten, vanadium and titanium.

In order to more clearly demonstrate thesteel -of-this invention, a heat of steel was made in an electric arc .Bercent Carbon I 0.0.3 Manganese 1.12 Silicon 0:28 Chromium 8.21 Nickel 4.42 Molybdenum 2.53 Aluminum t; 1.02 Nitrogen 0102 8 Iron with incidental impurities Balance 1700 F. and about 2000 F. The steel is maintained at this temperature for a sufiicient periodof time in order to take the alloying elements, especially the carbon or carhides and the aluminum, within solution within the taustenite phase. Preferably, the steel is then cooled at a Irate sufficiently fast to retain these elements in solution. Where the steel is in the form of sheet, air cooling will usually suffice; however, if the steel is heat treated .in the form of heavier sections, for example, bars, it may be desirable to subject the steel to a more severe quenching medium, for example, water or 'oil. Examination of the microstructure of the steel in the quenched condition from the solution heat treated temperature reveals a substantially completely martensiticstructure having "a grain size varying between ASTM Nos. 6 and "9. The steel may be hardened by subjecting it to an aging treatment at'a temperature in the range between about 900 F. and about 1100 F. for a time period ranging between '1 hour and 24 hours and thereafter air cooling. The steel in the aged condition exhibits excellent strength and ductility. Reference is directed to Table I'I which illustrates the mechanical properties of the steel having a composition set forth hereinbefore.

Table II TENSILE PROPERTIES (BAR) Test Hardness, Ten. 0.2% Yield Elong., Bed. of Condition Temp., Strength, Str., p.s.i. Percent Are F. p.s.1. Percent (I; quenched trom 1,800 F R.T. 31 151,870 114, 890 15.0 59. 9 (1 +Age at; 1,000 F.-4 HrS.--A.C R.T. 42 201, 610 184, 720 14. 3 55. 4 as s as as. at s 42 162:840 1451 360 11I 9 5710 l, 000 42 133, 380 112, 900 20. 0 66. 3

From the test results recorded in Table II, which tests were performed on material in the form of bars, it is apparent that after aging the steel of this invention exhibits excellent hardness at temperatures of up to 1000 F. Note, also, that tests at the elevated temperatures indicate that the steel has excellent strength and ductility as measured by the percent elongation and reduction of area. Substantially similar results were obtained when the steel of the same composition was tested in the form of sheet.

Reference is directed to Table II which lists the mechanical properties of the steel in sheet form.

Table III TENSILE PROPERTIES (SHEET) Test Hardness, Ten. 0.2% Yield Elong., Condition Temp., Rs Strength, Stu, p.s.i. Percent F. p.s.1.

1,800 F.l5 m1n.-A.O.+l,000 F.-4 hrs-AD.-- 3.1. 44. 5 202, 600 183. 490 5.0 1,800 F --5 min --A C.+1,000 F -4 hrs A 900 140, 600 126, 860 11. 0 1,000 F 5 min A C.+1,000 F 4 hrs A 0.... R11. 44. 6 203, 250 182,000 l5. 0 900 F -5 min -A O.+1,000 F 4 hrs A C- 900 143, 360 127, 780 9. B 2,000 F 5 min A O.+1,000 F -4 hrs A O-- R.T. 43 197, 620 179, 520 4. 5 2,000 F.-5 m1n.--A.O.+1,000 F.4 hrs.-A.O Q00 141, 300 121, 270 12. 5

From the test results recorded in Table III, it is apparent that the steel of this invention is also quite useful in sheet form, there being only a slight difference in the attainable mechanical properties exhibited by the steel in the form of sheet or in the form of bars.

The steel of this invention was subjected to corrosion testing in a substantially 100% humid atmosphere at 95 F. and exhibited only a few small rust spots after 307 hours exposure. The stress corrosion properties are also excellent in that the steel was stressed to 140,000 p.s.i., that is, approximately 80% of the yield strength, and was thereafter tested in a salt spray. The specimen had a life of 258 hours in said test.

It is apparent that the steel of this invention has an excellent combination of strength and ductility and exhibits corrosion resistance properties approaching those of the conventional 12% chromium steels. The steel can be hardened to high strength by a low temperature heat treatment after fabrication and exhibits a strength comparable to some of the better known precipitation hardening alloys. However, the steel has a unique advantage in that it is not subject to any of the difficulties encountered due to the stability of the austenite which plagues some of the better known precipitation hardening alloys. The steel is especially suited for use where a high strength to weight ratio is desirable and where corrosion resistance, while necessary, is of not too great a concern. There are no special skills or equipment needed to produce or fabricate the steel of this invention and the steel is hardenable using the equipment presently employed in the trade.

We claim:

1. A martensitic age hardenable stainless steel consisting of, up to 0.04% carbon, up to 2.0% silicon, from 0.25% to 3.0% manganese, from 7.0% to 12.0% chromium, from 1.0% to 6.0% nickel, from 0.5% to 1.75% aluminum, up to 5% of metal selected from the group 3. A martensitic age hardenable stainless steeleonsisting of, about 0.03% carbon, about 1.12% manganese, about 0.28% silicon, about 8.21% chromium, about 4.42% nickel, about 2.53% molybdenum, about 1.02% aluminum, about 0.028% nitrogen, and the balance iron with incidental impurities.

4. A precipitation hardened article of manufacture suitable for use at temperatures up to 1000 F. and formed of a martensitic stainless steel consisting of, up to 0.04% carbon, up to 2.0% silicon, from 0.25% to 3.0% manganese, from 7.0% to 12.0% chromium, from 1.0% to 6.0% nickel, from 0.5% to 1.75% aluminum, up to 5.0% of metal selected from the group consisting of molybdenum, columbium, tungsten, vanadium and titanium, up to 0.05% nitrogen, and the balance iron with incidental impurities, and which has been quenched from a solution heat treatment temperature between 1800' F. and 2000" F. and aged at a temperature in the range between 900 F. and 1100 F. for a time period ranging between 1 and 24 hours.

5. An article of manufacture suitable for use at temperatures of up to 1000 F. formed from a martensitic age hardenable stainless steel consisting of, up to 0.04% carbon, up to 2.0% silicon, from 0.25% to 3.0% manganese, from 7.0% to 12.0% chromium, from 1.0% to 6.0% nickel, from 0.5% to 1.75% aluminum, up to 5% of metal selected from the group consisting of molybdenum, columbium, tungsten, vanadium and titanium, up to 0.05 nitrogen, and the balance iron with incidental impurities.

References Cited in the file of this patent UNITED STATES PATENTS Foley May 10, 1955 Walton et a1. June 30, 1959 

1. A MARTENSITIC AGE HARDENABLE STAINLESS STEEL CONSISTING OF, UP TO 0.04* CARBON, UP TO 2.0% SILICON, FROM 0.25% TO 3.0% MANGANESE, FROM 7.0% TO 12.0% CHROMIUM, FROM 1.0% TO 6.0% NICKEL, FROM 0.5% TO 1.75% ALUMINUM, UP TO 5% OF METAL SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM, COLUMBIUM, TUNGSTEN, VANADIUM AND TITANIUM, UP TO 0.05% NITROGEN, AND THE BALANCE IRON WITH INCIDENTAL IMPURITIES. 